1. Field of the Invention
The invention relates to a transparent rare-earth oxide sintered body having at least one of yttrium, lutetium and ytterbium, that is used in discharge lamp tubes, laser host materials and materials for plasma observation corrosion-resistant windows of semiconductor producing devices and a manufacturing method thereof.
2. Description of the Background Art
So far, as a manufacturing method of a transparent yttrium oxide sintered body, a transparent lutetium oxide sintered body or a transparent ytterbium oxide sintered body, a technology where a molded body that contains, in terms of metal, aluminum in a range of 5 to 100 wt ppm and silicon of 10 wt ppm or less in one of yttrium, lutetium and ytterbium oxide is sintered at a temperature of 1450° C. or more and 1700° C. or less for 0.5 hr or more in a hydrogen, inert gas, mixed gas of hydrogen and inert gas or vacuum atmosphere is known (for instance, Japanese Patent Unexamined Publication JP-A-2003-89578).
Furthermore, a technology where a mixture of a calcium compound that becomes calcium oxide by pyrolysis in the range of 100 ppm to 4% to yttrium oxide, a zirconium compound that becomes zirconium oxide by pyrolysis in the range of 200 ppm to 10% to yttrium oxide and yttrium oxide of which primary particle diameter is 0.5 μm or less is molded and sintered is known (for instance, Japanese Patent Examined Publication JP-B-2939535).
In a method where an aluminum content is set in the range of 5 to 100 wt ppm in terms of metal, which is described in JP-A-2003-89578, assuredly, a transparent yttrium oxide sintered body, a transparent lutetium oxide sintered body or a transparent ytterbium oxide sintered body can be obtained. However, during the sintering, aluminum tends to segregate in grain boundaries of a yttrium oxide sintered body, a lutetium oxide sintered body or a ytterbium oxide sintered body. Accordingly, in order to inhibit aluminum from segregating, a sintering temperature is necessarily set at a temperature of 1700° C. or less. A sintering temperature of 1700° C. or less is low as a sintering temperature for yttrium oxide, lutetium oxide and ytterbium oxide that are high in the melting point. Accordingly, in order to sufficiently densify at the temperature, in place of a commercially available general-purpose yttrium, lutetium or ytterbium oxide powder, for instance, it is necessary that an aqueous solution of yttrium sulfate, lutetium sulfate or ytterbium sulfate, or yttrium nitrate, lutetium nitrate or ytterbium nitrate, or yttrium oxalate, lutetium oxalate or ytterbium oxalate is neutralized under a controlled temperature and speed to form yttrium hydroxide, lutetium hydroxide or ytterbium hydroxide, the hydroxide is calcined in a controlled atmosphere and temperature, followed by pulverizing, and thereby a raw material excellent in the sinterability is particularly prepared. This is generally not an easy process.
Furthermore, in order to render a silicon content such low as 10 wt ppm or less, a low silicon content raw material has to be prepared particularly. This is neither easy matter.
In addition, in a method according to JP-B-2939535, a primary particle diameter of yttrium oxide is necessarily rendered 0.5 μm or less. A process of finely pulverizing and a special raw material synthesis under severely controlled conditions are necessary. That is, there is a problem in that a general-purpose yttrium oxide powder cannot be used.
Still furthermore, in the JP-B-2939535, since in-line transmittance (the linear transmittance) of a transparent yttrium oxide sintered body having a thickness of 1 mm is only 40 to 60% to light of a wavelength of 500 nm, the transparency is not sufficient. Furthermore, there is another problem in that, because of a problem of contamination, yttrium oxide containing calcium that is an alkaline earth metal cannot be applied to a window material for a semiconductor producing device.